cpufreq: Move dev_pm_opp_{init|free}_cpufreq_table() to pm_opp.h

Move the declaration of functions defined in the OPP core to pm_opp.h.
These were added to cpufreq.h as it was the only user of the APIs, but
that was a mistake perhaps. Fix it.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>